fix typo

fix undefined error reported here.
https://github.com/adrahon/vagrant-kvm/issues/201#issuecomment-40050196

Signed-off-by: Hiroshi Miura <miurahr@linux.com>
Merge pull request #211 from adrahon/fix_env_typo_201_on_v0.1.5-maint

v0.1.6:  fix undefined env error